1. Field of the Invention
The present invention relates to an apparatus which performs at least either recording or reproduction of information to/from optical information recording medium to which information is recorded using holography.
2. Description of the Related Art
Conventionally, in holographic recording for recording information onto a recording medium using holography, in general, information light carrying image information and recording-specific reference light overlap with each other in a recording medium and interference pattern generated thereby is written onto the recording medium. For reproducing the recorded information, reproduction-specific reference light is irradiated onto the recording medium so as to reproduce image information by diffraction of the interference pattern (see Japanese Unexamined Patent Publication No. 11-311938).
Recently, for achieving super high density optical recording, volume holography, especially digital volume holography, has been developed to be in practical use and has attracted attention. The volume holography is a method in which the interference patterns are written three-dimensionally by actively utilizing the thickness direction of the recording medium. In the volume holography, the diffraction efficiency is improved by increasing the thickness and the storage capacity can be increased by using multiple-recording. The digital volume holography is a computer-oriented holographic recording method in which image information to be recorded is limited to binary digital patterns, while using the same recording medium and the recording system as that of the volume holography. In the digital volume holography, for example, picture information such as an analog picture is once digitized to be two-dimensional digital pattern and then it is recorded as image information. The digital pattern information is read out and decoded to be displayed as the original picture information. Thereby, even when SN ratio (signal-to-noise ratio) at the time of reproduction is not so good, it becomes possible to reproduce the information which notably faithfully reflects the original information by carrying out differential analysis or carrying out error correction by coding the binary data.
As the holographic recording, an effective method is the one which uses optical pickup devices including an optical system for recording/reproducing information to/from a recording medium by employing a disk-type recording medium such as a CD (compact disk), a DVD (digital versatile disk) and the like.
In general, in an optical disk storage device, a focus servo and a tracking servo are operated by driving an objective lens inside the optical pickup device through rotating a disk-type recording medium. In such a configuration, angle of incident of the light irradiated onto the recording medium from the optical pickup device is altered when the disk-type recording medium is deflected or deformed. However, a CD, a DVD or the like has a configuration in which the magnitude of light reflected by a pit can be read out so that changes in the angle of incident are not significant as long as the light is irradiated onto the pit.
However, in the case of holographic recording, the interference pattern between information light and recording-specific reference light is recorded for recording and diffraction of reproduction-specific reference light and the interference pattern is reproduced for reproduction. Thus, if the information light and each reference light at the time of recording and reproduction do not enter at the same angle, the S/N ratio becomes notably deteriorated.
Therefore, when making the holographic recording fit into a practical use, a device for correcting a relative inclination between the recording medium and the optical pickup device becomes important. Conventionally, there is a method for controlling the angle of incident so that light enters a recording medium always at the same angle by tilting a pickup main body of an optical pickup device according to the relative inclination between the recording medium and the optical pickup device (see Japanese Unexamined Patent Publication No. 2001-273650).
However, the pickup main body includes a light source for generating light, a means for generating information light which carries information by spatially modulating the light from the light source, a means for generating recording-specific reference light and reproduction-specific reference light, various optical systems and an objective lens. Thus, when tilting the pickup main body according to the inclination of the recording medium, a large-scaled driving system for tilting is required. Therefore, it is not suitable for achieving size-reduction.
Further, by tilting the pickup main body, the focal point position of the objective lens is altered. Thus, it is necessary to correct the position, thereby complicating the control.